This invention relates to the field of integrated circuit fabrication. More particularly, this invention relates to fabrication of electronic organic substrates on which monolithic integrated circuits are mounted.
Electronic organic substrates, also called printed circuit boards or package substrates, are used as a platform for monolithic integrated circuits, such as array products like ball grid arrays, to which the monolithic integrated circuit is mounted and encapsulated.
The package substrate is typically formed of several electrically conductive layers, sandwiched between non electrically conductive layers and a non electrically conductive core. The electrically conductive layers, typically formed of a metal such as copper, are used for power and signal routing. The non electrically conductive core provides structural strength to the substrate, and reduces damage to the package from stresses such as those induced by mechanical load and thermal energy.
Typically, a copper layer is deposited on the core material and then a layer of photo imageable ink is applied to the copper layer. A pattern is exposed on the photo imageable, to expose portions of the copper layer, which are then etched away. A layer of solder mask material is then applied over portions of the patterned copper layer, and additional electrically conductive materials, such as nickel and gold, are plated on the exposed portions of the patterned copper layer. A final layer of solder mask may then additionally be applied.
Unfortunately, there are several problems with the present methods. For example, there is no current method for directly controlling the width of the materials that are plated on the copper layer, as they can grow out sideways from the copper layer across the substrate. Thus, the features formed in this manner can only be so close together, or they may grow together and form electrically conductive bridges during the plating process. In addition, the non electrically conductive solder mask is typically applied in a screen printing operation. Because geometries can only be reduced to a certain level with screen printing, designers are not able to define electrically conductive traces that are as fine or as close together as may be desired.
What is needed, therefore, is a method of fabricating electronic organic substrates that overcomes at least some of these and other problems.
The above and other needs are met by a method of forming electrically conductive elements on a base layer of an electronic substrate without the use of solder mask. A layer of electrically conductive material is deposited on the base layer, and a first layer of photo imageable ink is applied over the electrically conductive material layer. The first layer of photo imageable ink is patterned to expose portions of the electrically conductive material layer, which are then etched to resolve traces in the electrically conductive material layer. The first layer of photo imageable ink is removed, and a second layer of photo imageable ink is applied over the traces and channels between the traces. The second layer of photo imageable ink is then patterned to expose the traces, and a third layer of photo imageable ink is applied over the traces and the second layer of photo imageable ink. The third layer of photo imageable ink is patterned to expose deposition sites on the traces, within which are formed electrically conductive fingers. Both the second layer and the third layer of photo imageable ink are retained on the electronic substrate.
In this manner, the second layer of photo imageable ink is used to constrain the lateral growth of the electrically conductive fingers within the deposition sites, and the traces can be formed at a finer pitch. In addition, because the photo imageable ink can be patterned at a finer resolution than the solder mask can be screen printed, the traces can be formed at an even finer pitch. Thus, the improvements of the present invention enable the fabrication of an electronic substrate having conductive elements that are placed at a finer pitch.
In various preferred embodiments there is an additional step of leveling the second layer of photo imageable ink with the traces prior to applying the third layer of photo imageable ink. Preferably the base layer comprises at least one of bismaleimide triazine, flex circuit, and FR4. The electrically conductive material preferably comprises copper, and is most preferably formed to a thickness of between about six mils and about eighteen mils. However, thicker copper tends to be better, and even greater thicknesses may be used. The step of forming electrically conductive fingers preferably comprises first plating a layer of nickel and then plating a layer of gold within the deposition sites. The bottom layer preferably has a thickness of between about five microns and about fifteen microns, and the top layer preferably has a thickness of between about one half micron and about one and one half micron. Preferably, the first layer of photo imageable ink, the second layer of photo imageable ink, and the third layer of photo imageable ink are all formed of a photo imageable ink, such as is commonly available. In one embodiment the base layer is singulated into several concurrently fabricated electronic substrates.